A nitride semiconductor has characteristics such as high saturation electron velocity and wide band gap. Thus, various studies have been conducted regarding an application of the nitride semiconductor to a semiconductor device with high withstand voltage and high output power, by utilizing these characteristics. For example, a band gap of GaN being one kind of the nitride semiconductor is 3.4 eV, which is larger than a band gap of Si (1.1 eV) and a band gap of GaAs (1.4 eV). Accordingly, GaN has high breakdown electric field intensity, and thus is quite promising as a material of a semiconductor device for power supply which obtains high voltage operation and high output power.
As a semiconductor device with a nitride semiconductor, there have been made numerous reports on a field effect transistor, particularly a high electron mobility transistor (HEMT). For example, an AlGaN/GaN-HEMT using GaN as a channel layer and AlGaN as a carrier supply layer is attracting attention among GaN-based HEMTs. A strain resulted from a lattice mismatch between GaN and AlGaN occurs in AlGaN in the AlGaN/GaN-HEMT. Then, two-dimensional electron gas (2DEG) of high concentration is obtained from piezoelectric polarization caused by the strain and spontaneous polarization of AlGaN. Accordingly, the AlGaN/GaN-HEMT is expected as a high efficiency switching element and a high-withstand-voltage electric power device for electric vehicle, or the like.
However, it is quite difficult to form a GaN substrate with excellent crystallinity. Thus, conventionally, a GaN layer and an AlGaN layer and the like are formed mainly over a silicon substrate, a sapphire substrate or a silicon carbide substrate through heteroepitaxial growth. It is easy to obtain, in particular, a silicon substrate with large diameter and high quality at a low cost. Accordingly, a research regarding a structure in which a channel layer and a carrier supply layer are grown over a silicon substrate has been extensively conducted.
However, it is difficult to suppress a leak current that flows between a drain electrode and the semiconductor substrate in a conventional GaN-based HEMT with a semiconductor substrate such as a silicon substrate. A maximum operating voltage (maximum operating withstand voltage) of the HEMT is limited due to the leak current.
Patent Literature 1: Japanese Laid-Open Patent Publication No. 2009-026975
Patent Literature 2: Japanese Laid-Open Patent Publication No. 01-096964